In the past, as an exhaust gas purification apparatus arranged in an exhaust passage of an internal combustion engine, there has been developed one in which a filter that supports thereon an NOx selective reduction catalyst (hereinafter, referred to as an SCR catalyst) (see, for example, Patent literature 1). The filter traps particulate matter (hereinafter, referred to as PM) in exhaust gas. The SCR catalyst serves to reduce NOx in the exhaust gas by using ammonia (NH3) as a reducing agent. Hereinafter, a filter supporting such an SCR catalyst thereon may also be referred to as an SCRF.
By adopting the SCRF as an exhaust gas purification apparatus, the size of the exhaust gas purification apparatus can be made smaller in comparison with the case where the filter and the SCR catalyst are separately arranged in the exhaust passage. As a result, mountability of the exhaust gas purification apparatus can be improved. In addition, by the adoption of the SCRF, it becomes possible to arrange the SCR catalyst at a location more upstream in the exhaust passage. At the more upstream side in the exhaust passage, the SCR catalyst is arranged, the easier it becomes for the SCR catalyst to be heated by the heat of the exhaust gas. For that reason, it is possible to attain an improvement in the warm-up performance of the SCR catalyst as well as an improvement in the rate of NOx reduction in the SCR catalyst.
Here, the PM trapped by the SCRF deposits or accumulates therein. For that reason, in an exhaust gas purification system provided with the SCRF, filter regeneration processing is carried out. The filter regeneration processing is to oxidize and remove the PM deposited in the SCRF. The filter regeneration processing is achieved by supplying fuel to a pre-catalyst having an oxidation function which is arranged in the exhaust passage at the upstream side of the SCRF. When the fuel is oxidized in the pre-catalyst, the exhaust gas flowing into the SCRF will be heated by the heat of oxidation. For that reason, the temperature of the SCRF can be raised to a filter regeneration temperature at which the oxidation of the PM is promoted.